1. Field of the Invention
The present invention relates to an optical information recording medium, and more particularly relates to an optical information recording medium having at least one information recording layer and a light-transmitting layer on a main surface of a substrate.
2. Description of the Prior Art
Recently, research relating to various types of optical information recording has been studied in the optical information recording field. Optical information recording methods have been emerging which are applicable to a range of uses, because these methods can allow media to have higher density, can record/reproduce information by a non-contact method, and can also achieve these objectives at a low price. Currently, an optical disk has a structure produced, for example, by forming an information layer on a transparent resin layer with a thickness of 1.2 mm, and then covering and protecting the layer with an over coating, or by forming information layers on one or both sides of a transparent resin layer with an over coating, or by forming information layers on one or both sides of a transparent resin layer with a thickness of 0.6 mm, and then laminating two of the information layers.
Recently, as a way to increase the recording density of optical disks, methods have been studied such as increasing the numerical aperture (NA) of an objective lens, and shortening the wavelength of the laser. In these methods, if the thickness of a recording/reproducing carrier (which is a substrate of the side on which an optical laser is incident) is thin, the influence of an aberration of the laser spot decreases, and allowance for a gradient angle (tilt) of a disk increases. From this, an idea was proposed to set the thickness of the recording/reproducing carrier to be around 0.1 mm, NA to be around 0.85, and the wavelength of a laser to be around 400 nm (see Japanese unexamined patent publication H08-235638, for example). Here, considering effects on the focus of the recording/reproducing light and risk of spherical aberration, it is preferable that the thickness variation of the recording/reproducing carrier is reduced to be within 5%. Even in an optical disk having such reduced thickness variation, the recording/reproducing carrier of which has thickness is 0.1 mm, a thickness of the disk is preferably 1.2 mm like a conventional CD or DVD because the disk should have compatibility with existing hardware.
An optical disk having a recording/reproducing carrier with a thickness of 0.1 mm has an asymmetrical structure in the thickness direction because the thickness of the disk is 1.2 mm. Because of the asymmetrical structure, warping of the optical disk occurs due to variations in environmental conditions such as temperature or humidity. More specifically, when placing the optical disk, which is at room temperature, in a recording/reproducing machine which has been heated to 50 degrees C. by uninterrupted operations, drastic warping occurs in a short time.